Neutrophil directional migration in response to chemical gradients, also known as chemotaxis, is one of the key phenomena in immune responses against bacterial infection and tissue injury. Alterations in neutrophil chemotaxis, e.g., as a result of burns or trauma, may lead to chronic inflammation and further tissue damage. Identifying alterations of neutrophil chemotaxis may help estimate the risk for infections more accurately. To better study neutrophil chemotaxis, in vitro assays have been developed that replicate chemotactic gradients around neutrophils. Since red blood cells out-number neutrophils and have the propensity to clot, measurements of neutrophil migration pattern in whole blood is challenging. For this reason, traditional assays (e.g. transwells) uses isolated neutrophil cells from whole blood.
However, existing assays can include one or more drawbacks. For example, such assays require time-consuming processing of blood to isolate neutrophils. Furthermore, such isolation may alter the responsiveness of neutrophils compared to in vivo conditions, leading to inaccurate characterization of the neutrophils. For instance, certain chemotaxis assays utilize cell separation methods, such as positive selection or negative selection, which are prone to activating neutrophils by engaging specific receptors on the neutrophils. Once activated, the neutrophils' migration profile can be altered; however, this change may not be directly related to the biological condition of interest, but rather a response from the applied stress introduced by cell isolation protocols. Additionally, existing assays require processing relatively large volumes of blood. These assays lack accuracy and/or require use by technicians having specialized training. These limitations, among others, restrict the assays' usefulness in clinical laboratories.